X86: Add a description of the Intel Atom Silvermont CPU.

Currently this is just the atom model with SSE4.2 enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189669 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Check for zero shift amounts before subtracting one causing integer overflow.

PR17026. Also avoid undefined shifts and shift amounts larger than 64 bits
(those are always undef because we can't represent integer types that large).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189672 91177308-0d34-0410-b5e6-96231b3b80d8
Checking commit access; removed one space added in previous test checkin by Jim

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189673 91177308-0d34-0410-b5e6-96231b3b80d8